1. Field of the Invention
This invention relates to a process for preventing caking of ammonium bifluoride and/or ammonium flourids, whose particles readily cake together under the effect of temperature, pressure and water vapor.
2. Background Information
Ammonium bifluoride/fluoride (commercial mixture of NH.sub.4 HF.sub.2 and NH.sub.4 F) is produced industrially by the reaction of gaseous ammonia and hydrogen fluoride. Scales, pastilles and granules can be produced from the solidified melt with suitable apparatus. It is also known that ammonium bifluoride/fluoride salt mixtures can be obtained from aqueous solutions by crystallization and filtration followed by drying (Ullmann, (4) 11, page 621). These products have the disadvantage that in storage they tend to cake and/or clump together when subjected to pressure, e.g., when sacks of the product are stacked on pallets, or to temperature, e.g., during prolonged storage in regions with a high annual average temperature, and/or to moisture. In extreme cases, the packaged product may harden to an agglomerate which must then be broken down again into a pourable, fine grained product by mechanical means or discarded as completely unusable.
It has already been attempted to solve the aforementioned means of additives. It has long been known that inorganic additives such as kieselguhr (Winnacker-Kuchler, (3) 2, page 512), Clay (Ullmann, (4) 23, page 322), various types of talc (Kirk-Othmer, 19, pages 608 to 614) and/or calcium carbonate (Kirk-Othmer, (3) 14, page 377) can be used as anticaking and fluidizing agents for ammonium compounds, but they must be used in relatively large quantities (1 to 5% by weight) and have the disadvantage that they prevent caking which is only due to moisture. Such additives have no effect against the action of pressure and temperature. Another disadvantage of these additives is that since they are virtually insoluble in water, they interfere with process steps involving crystallization of a product from aqueous solutions and in some cases even react with the salts, e.g., with ammonium bifluoride. Thus when kieselguhr is added to ammonium bifluoride solutions, for example, toxic gaseous SiF.sub.4 is released, and when calcium compounds are added they bind fluoride and thereby reduce the yield of ammonium bifluoride.
It is also known that ammonium compounds can be prevented from caking and clumping together by the addition of dyes. GB Pat. Nos. 625 077; 665 478 and 743 602 describe the addition of aniline red to ammonium salts. This dye has, however, the disadvantage that it is active only at low temperatures, at which the tendency to caking is in any case less marked. Further, it produces an intense color in the treated material.
The problem therefore existed of providing anti-caking agents and a process by which ammonium bifluoride and/or fluoride salt mixtures and salts could be modified with these anti-caking agents so that the salt or salt mixture would not agglomerate to clumps even when stored at elevated ambient temperatures and under the action of pressure and/or moisture and would remain in a loose, pourable state as crystals, scales, pastilles or granules.